In a spark plug that has a small sized center electrode, enhancement of corruption-resisting property and heat-resisting property is promoted. The spark plug 1 includes a center electrode 5, an insulator 2 and a metal shell 3, and the insulator 2 includes a foot section 13, a tapered section 14 and a middle body section 12. The maximum outer diameter of a portion, which is arranged within the foot section 13, of the center electrode 5 becomes 3.0 mm or less. A step section 21 and a leading end side inner periphery section 51 are formed in the inner periphery of the metal shell 3 and the tapered section 14 is engaged to a step section 21. When A (mm3) is a volume of a portion of 2 mm of the insulator 2 from leading end of the insulator 2 toward the rear end side along the axis CL1 and B (mm3) is a volume of a portion of the insulator 2 from the rear end of the portion of the tapered section 14 which is engaged to the step section 21 to the leading end side and the portion is 1.5 mm or less of the diameter difference between the leading end side inner periphery section 51 and its outer periphery portion, 0.12≦A/B≦0.24 is satisfied.